Process for polymerizing lower alkyl esters of acrylic acid



i atented Feb. 27, 1940 UNITED STATES PROCESS FOR. POLYMERIZING LowmtALKYL ESTERS or ACRYLIC ACID John William Croom Crawford, Frodsham,England, and James McGrath, Ardrossan, Scotland, assignors to ImperialChemical Industries Limited, a corporation of Great Britain No Drawing.Application May 5, 1938, Serial' No. 206,277. 1933 15 Claims.

This invention relates to the production of plastic materials by thepolymerization of the esters of acrylic acid and more particularly tothe polymerization of methyl acrylate in granular form. The applicationisa continuation in part of our copending application 749,500 filedOctober 22,1934.

It has already been proposed to polymerize derivatives of acrylic acid,or their homologues, or vinyl esters, by emulsifying such derivatives inliquids in which they are substantially insoluble and subsequentlypolymerizing them. such previous proposals, however, the unsaturatedester compounds were generally first emulsified and then subjected topolymerizing conditions. Proceeding by these method involves a numberof. steps: (1) the addition to the monomer containing a polymerizationcatalyst of the emulsifying agent and la nonsolvent for the monomer, (2)emulsifying the resulting mixture, (3) polymerizing the emulsion, (4)adding coagulating agent, (5) coagulating the emulsified polymer, (6)removing the coagulating agent. The ultimate product obtained by such apro- 25 cedure, furthermore, is not generally in a form which is easilydissolved, molded, or otherwise fabricated into an article of commerce.

We. have now discovered that unsaturated esters, such as the, loweralkyl esters of acrylic acid or its homologues or mixtures of the samecan be caused to yield polymerization products in granular or globularform, providing the polymerization is eflected in the presence of asuitable liquid medium and with proper agitation whereby agglomerationof the individual droplets, on the one hand, and, their extendeddispersion, on the other, are prevented.

Moreover, many of the disadvantages inherent in the emulsionpolymerization process are eliminated by our process. It involves noemulsification prior to polymerization; no addition of coagulating agentfor the product of our invention is free settling; no coagulation step,and hence no coagulating agent to remove; and most important the freesettling product of our invention is in such form that it may be readilywashed, dissolved, molded or otherwise treated for use in the variousarts for which it is adapted. Furthermore, as would be expected, becauseof its high state of subdivision, the polymer obtained by the emulsionprocess has a low bulk density which is not readily adapted tocommercial molding and considerable difliculty is encountered inattempting to pellet or otherwise compress this product. Incontradistinction the In Great Britain October 25,

granular product of this application has a high bulk density and ismolded with facility.

Generally speaking, the production of the polymerization product ingranular or globular form is effected by adding the monomericunsaturated 5 compound to a non-solvent liquid therefor containing acolloid, the resulting mixture is stirred, its temperature raised, andthe stirring, amount of colloid, and temperature maintained at theproper degree until polymerization is complete. 10 When the stirring isstopped the polymerized resin settles freely and substantiallyinstantaneously, the supernatant liquid is decanted, and the product iswashed and dried. The finished product consists of fine sphericalgranules, vary- 15 ing in size from $4; to 5 mm. in diameter accordingto the conditions of polymerization, in contradistinction to the slimyproduct resulting from the coagulation of the emulsion polymer. Toobtain such a unique and surprisingly useful 0 polymerization product bythe process that will hereinafter be more fully particularized is, inview of the prior art, most unexpected.

When polymerizing in accord with the general method hereinbeforedisclosed, it will be 26 found that the form of the polymerized resinwill vary in accord'with degree of stirring, and concentration ofcolloid. The feature of the present invention is to so regulate thesefactors that a product in granular or globular form is 30 obtained whichis readily removed from the polymerizing solution and may be easilyfiltered and washed. It will be appreciated that, if the rate ofstirring of a given system containing a monomer, a non-solvent therefor,and a colloid is suf- 86 flciently great, an emulsion will result, andit has been found that upon polymerization thereof there is obtained apermanent suspension of minutely small particles. Such a product is thencoagulated to a slime which is most diflicult 40 to wash to removecoagulating agent. Alternatively, should the stirring of a given systembe carried out at a very moderate rate, very little,if any,emulsification or dispersion of the monomer will be effected and inmanyinstances the monomer upon polymerizing will form ahard horny solidlayer which sticks tenaciously to the vessel and stirrer and can beremoved therefrom only with great difliculty.

In lieu of varying the rate of stirring the amount. of colloid may, ofcourse, be varied. Assuming now for the moment that an optimum stirringrate be employed, it is conceivable, and has been found by experiment tobe actually a fact, that if a large amount of the colloid be employed,say, for example, 4% of glycol cellulose, a product similar to thatobtained by very vigorous stirring will result, i. e. an emusionpolymer, while, on the other hand, should concentrations of this colloidbe maintained at in the neighborhood of, say 0.01%, a product comparableto that of mild stirring will be obtained.

We have found, however, that by following the directions hereinafter tobe specified, it is possible to so control the above factors that thepolymerized product will be in a granular or globular form, which isvery easily filtered and washed from the dispersing medium. As has beenindicated, the stirring should not be too violent nor the amount ofcolloid so great that after completing the polymerization a permanentemulsion of the polymer results.

Accordingly, in order to obtain the product of this invention we havefound that stirring should be used in amount substantially equivalent tothat obtained in an ovaled bottom enameled vessel containing no baflleshaving a capacity of 50 gallons and provided with stirrer of a flatinverted T type (10" x 1%"), occupying 0.4 diameter of the pot andimmersed 10 inches in the solution, the vessel having been charged with25 gallons of Water containing 0.3% of glycol cellulose and 5 gallons ofmonomeric methyl methacrylate, the stirrer being revolved at 510 R. P.M. and the solution maintained at a temperature of 82 C. It will befound that after a period of approximately one hour, under theseconditions, the monomer will be substantially entirely converted to apolymer which is in granular form and which settles freely and quicklyupon stopping the stirrer. If in this example the concentration ofglycol cellulose be increased to 4%, an emulsion is obtained that givesa slimy product upon coagulation. On the other hand, should the amountof glycol cellulose be lowered to approximately 0.01% the polymerizationwill proceed with simultaneous fusion of the particles of polymer toeach other, giving a solid, horny mass, which will adhere firmly to thevessel and often stop the stirring mechanism entirely, and necessitatedestruction of the vessel in many instances before the resin can beremoved therefrom.

An object of this invention, therefore, is to, on the one hand, conductthe polymerization of the herein named bodies under such conditions andconcentrations that the emulsified product or alternatively the solid,horny mass is not obtained. Between these extremes, then, liesapplicants process and they have found that by proper coordination ofstirring and character and concentration colloid, it is possible to keepbetween the two undesirable limits and obtain the preferred product.

According to the present invention, plastic materials may be obtained ingranular or globular form by polymerizing, in the manner hereinbeforeindicated, alkyl esters of methacrylic acid or mixtures of the same, andmore particularly methyl, ethyl, propyl, butyl, isobutyl, and the highermethacrylates and including likewise the various saturated andunsaturated esters of methacrylic acid disclosed in the United Statespatents of H. J. Barrett and D. E. Strain Nos. 2,129,662-3-4-5-6-7, theUnited States patent of J. C. Woodhouse, 2,129,722, the United Statespatent of G. D. Graves, 2,129,685, and the United States patent of E. F.Izard, 2,129,694, patented on September 13, 1938. Mixtures andinterpolymers of these various unsaturated compounds with each other orwith other polymerizable compounds such as the halogenated vinyl esters,etc. may likewise be polymerized by the process of the presentinvention.

As has been indicated, the proportions of the colloids will vary inlarge measure in accord with I their ability to act as a dispersingagent, but generally speaking lyophilic colloids are preferred. By theterm lyophilic colloid as employed herein, we mean to include a highmolecular weight material which has an aflinity for the dispersingmedium. Compounds such as soluble starch, gum tragacanth, gum acacia,gum tragon, water soluble glycol cellulose, sodium alginate, agar agar,glue and gelatin may be used. Glycol celluloses which are soluble insodium hydroxide, altho insoluble in water, may beused in very dilutealkaline solutions. Ordinarily, these colloids and colloids having likedispersing powers may be used in concentrations varying from .2 to 3%,altho in some instances more may be required,

the percentage being based upon the weight of the unsaturated compoundbeing polymerized. Those skilled in the art will appreciate the factthat the enumerated colloids all do not have equal dispersing powers, e.g. gum tragacanth is less effective than glycol cellulose and varioustypes of starches vary in accord with the past history of the starch.Accordingly, when changing from one colloid to another a greater orlesser amount thereof must be employed providing the effectiveness ofthe substituting colloid is less or greater than that of the colloidfirst employed. The determining factor is, of course, that thepercentage of colloid be so adjusted that our preferred granular form ofpolymer is produced.

The volume of the aqueous or other medium in which the polymerization isto be effected may conveniently amount to about one to ten times thevolume of the unsaturated compound, and the reaction mixture may beprepared by adding the aqueous or other type of solution to theunsaturated compound or vice versa. The former method is preferred.

By what has been said it will be appreciated that agitation is essentialfrom the inception of the actual polymerization until the product issubstantially completely polymerized, since the medium in which theunsaturated compound is treated is of such a type that separation in twoliquid phases takes place if the agitation is interrupted before orshortly after the inception of polymerization.

The process of this invention results in the production of thepolymerized material in the form of solid granules or .globules whichare chemically and physically homogeneous, altho the particles may ormay not be of such small diameter that the material has a powder-likeappearance in bulk. This product, as has hereinbefore been emphasized,is particularly easy to remove from the polymerization vessel and may beapplied directly for any purpose for which the polymer is commonlyemployed without the necessity of previous grinding, cutting, or othercomminuting treatment.

The polymerization may be efiected, for instance, by heating the mixturein contact with a polymerization catalyst, or by subjecting it toultraviolet light, agitation being continued until the polymer is nolonger sticky. The polymer is then isolated from the reaction mixture,by filtration, and washed with an appropriate agent to remove as far aspossible any adhering colloid. It may then be washed with water anddried. If

desired, a proportion of plasticizer substantially insoluble in watermay be mixed with the unsaturated compound before commencing thepolymerization or should a non-solvent other than water be employed,plasticizers insoluble in such liquid medium may likewise be used. Bysuch a procedure a plasticized granular polymer will thus be produced.Similarly, dyed, tinted, or filled polymer may be obtained.

As the heat evolved in the polymerization of these compounds isfrequently large, a part only of the charge of the monomeric compoundmay be mixed with the aqueous or other medium at the commencement ofpolymerization, the remainer of the monomer being added graduallythereafter so as to maintain the evolution at a given rate. v

The invention is illustrated by the following examples in which allparts are by weight:

Example 1.-562 parts of a aqueous solution of a water soluble glycolcellulose were freed from any suspended insoluble matter by centrifugingand diluted with 5,000 parts of water. This solution was added to 2,270parts of methyl methacrylate containing 22.7 parts of benzoyl peroxidein solution and the mixture was placed in an enameled cast iron jacketedvessel, fitted with a reflux condenser, a thermometer, and a mechanicalstirrer with a water seal and external bearings to prevent oilcontamination of the product. After stirring had been commenced steamwas admitted to the jacket, and the temperature of the reaction mixtureraised to the neighborhood of 80 C. The speed of reaction was controlledby admitting water at a suitable temperature to the jacket so that aconvenient rate of refluxing of water-ester mixture was maintained. Whenthe bulk of the reaction had taken place, steam was re-admitted to thejacket and the temperature of the reaction mixture raised to theneighborhood of 100 C. to ensure complete polymerization of the methylmethacrylate.

The solid product was separated by filtration and washed firstly'with0.1% caustic soda solution to remove adsorbed glycol cellulose, andsubsequently with several successive quantities of water. The productwas then centrifuged and dried at a temperature of 40 0., being obtainedfinally in the form of a white sand-like powder.

A stirrer speed of about 250 revolutions per minute is suitable whentreating a charge of lbs. of the ester as described in the aboveexample.

Example 2.-5 parts of benzoyl peroxide and 50parts of dibutyl phthalatewere dissolved in 500 parts of methyl methacrylate, and the solution wasagitated with a solution of 24 parts of gum acacia in 1200 parts ofwater. The mixture was heated and polymerization carried out asdescribed in Example 1. The solid product was separated by filtration,washed by several successive quantities of water and dried. If desired,an additional washing with 0.1 per cent of hydrochloric acid may begiven directly after the filtration.

The product, a fine powder, is suitable for use in the production ofmolded articles.

Example 3.A solution of 8.45 parts of benzoyl peroxide in 845 parts ofethyl methacrylate was mixed with a solution of 211 parts of 8% aqueousglycol cellulose in 1,500 parts of water. Polymerization was effected asin Example 1, and the product which was similarly isolated formed awhite sand-like granular mass.

Example 4.-7.'l parts of a ruby-colored dye,

known under the term Spirit Red 111 (Color Index No. 258), and 40.9parts of benzoyl peroxide were dissolved in a mixture of 4.086 parts ofmethyl methacrylate and 408.6 parts of dibutyl phthalate. The solutionwas agitated and heated with 5 a solution of 3'74 parts of 8% aqueousglycol cellulose in 10,500 parts water. The polymer was purified as inExample 1, and formed a red granular powder suitable for use in theproduction of injection molded articles.

Example 5.--100 parts by weight of methyl methacrylate containing insolution 1 part of benzoyl peroxide were mixed with 200 parts of watercontaining in solution 2 parts of a suitable soluble starch. The mixturewas stirred at 1000 15 R. P. M. and steam was blown into the stirredmixture to raise the temperature to 75 C. and maintain it at that value.After 115 minutes the temperature began to rise and the steam was shutoff. When the maximum temperature of 82 C. was reached this mixture wascooled to 60 C. by the addition of cold water. The solid granularproduct was separated by centrifuging and washed with 1% HCl, water, 2%ammonia solution and repeatedly with water. The washed granules weredried at a temperature of 100 C.

Example 6.100 parts by weight of methyl methacrylate containing insolution 1 part of benzoyl peroxide were mixed with 200 parts of watercontaining in solution 2 parts of gelatine. 0 The mixture was treated inthe same manner as in Example 5 and a granular polymerization productwas obtained.

Example 7.--0.2 part of benzoyl peroxide was dissolved in 20 parts ofmethyl methacrylate and 5 parts of the solution added to 35 parts of an0.26% aqueous glycol cellulose solution. The mixture was placed in anenameled cast iron vessel fitted with a reflux condenser and an agitatorand heated to 80 C. with mechanical agitation, until the commencement ofvigorous polymerization was noticed. The rest of the benzoylperoxide-ester solution was added to the reaction mixture at such a rateas to allow of convenient control of the heat evolved in the reaction.The 65 granular product was washed with dilute (1%) caustic soda, andthen with water, finally being dried in an oven at about C. in a currentof air.

Example 8.5 parts of benzoyl peroxide were 0 dissolved in 500 parts ofvinyl acetate, and the solution stirred with 1,000 parts of 0.5% glycolcellulose solution, with warming. Polymerization occurred with vigorousevolution of heat. Monomeric vinyl acetate and water which boiled ofifrom the reaction mixture were condensed and returned to the system. Oncompletion of polymerization, the mixture was cooled down, and thegranular polymerized vinyl acetate drained from the aqueous portion,washed with cold dilute caustic soda, then with cold water, and spreadout in thin layers on trays to dry at atmospheric temperature.

Example 9.--A solution of 0.6 part of benzoyl peroxide in 2.92 parts ofmethyl methacrylate and 292 parts of methylacrylate was mixed with asolution of 1.2 parts of polymethacryl amide in 240 parts of water.Polymerization was effected as in Example 1, and the product which wassimilarly isolated formed a mass of soft, fairly tough granules.

Example 10.A solution of 0.6 part of benzoyl peroxide in 45 parts ofmethyl methacrylate and 14.4 parts of methyl acrylate was mixed with asolution of 1.2 parts of polymethacryl amide in 240 parts of water.Polymerization was efiected as i1- Example 1, and the product which wassimilarly isolated formed a mass of soft, fairly tough granules.

Example 11.-A solu'tion of 0.6 part of benzoyl peroxide in 60 parts ofmethyl acrylate was mixed with a solution of 0.6 part of methylatedstarch in 240 parts of Water. Polymerization was effected as in Example1, except that the refluxing temperature was lower, which necessitated alonger time to effect complete polymerization. The product was isolatedas in Example 1 and formed a mass 'of fairly soft granules.

From a consideration of the above specification it will be appreciatedthat many changes may be made in the invention as covered in theappended claims without departing from its scope.

We claim:

1. A process for the preparation of polymeric lower alkyl esters ofacrylic acid in granular form which comprises dispersing andpolymerizing while thus dispersed a monomeric lower alkyl ester ofacrylic acid in a non-solvent therefor by means of a dispersing agentand stirring, the concentration of the dispersing agent being notsubstantially greater than an amount suflicient to maintain the ester inthe dispersed phase while stirring is continued and to preventcoalescence of the dispersed particles during polymerizaticn; thequantity of dispersing agent also being insufilcient to give apermanently dispersed product.

2. A process for the preparation of a polymeric lower alkyl ester ofacrylic acid in granular form which comprises dispersing andpolymerizing while thus dispersed a monomeric lower alkyl ester ofacrylic acid in water by means of a dispersing agent and stirring, theconcentration of the dispersing agent being not substantially greaterthan an amount sufiicient to maintain the ester in the dispersed phasewhile stirring is continued and to prevent coalescence of the dispersedparticles during polymerization; the quantity of dispersing agent alsobeing insufficient to give a permanently dispersed product.

3. A process for the preparation of methyl acrylate polymer in granularform which comprises dispersing and polymerizing While thus dispersedmonomeric methyl acrylate in a nonsolvent therefor by means of adispersing agent and stirring, the concentration of the dispersing agentbeing not substantially greater than an amount suflicient to maintainthe esters in the dispersed phase while stirring is continued and toprevent coalescence of the dispersed particles during polymerization;the quantity of dispersing agent also being insuflicient to give apermanently dispersed product.

4. A process for the preparation of ethyl acrylate polymer in granularform which comprises dispersing and polymerizing while thus dispersedmonomeric ethyl acrylate in a nonsolvent therefor by means of adispersing agent and stirring, the concentration of the dispersing agentbeing not substantially greater than an amount suflicient to maintainthe esters in the dispersed phase while stirring is continued and toprevent coalescence of the dispersed particles during polymerization;the quantity of dispersing agent also being insuficient to give apermanently dispersed product.

5. A process for the preparation of methyl acrylate polymer in granularform which comprises dispersing and polymerizing while thus dispersedmonomeric methyl acrylate in water by means of glycol cellulose, theconcentration of glycol cellulose being not substantially greater thanthat necessary to maintain the monomeric ester in the dispersed phasewhile stirring is continued and to prevent coalescence of the dispersedparticles of the methyl acrylate during its polymerization; the quantityof dispersing agent also being insuflicient to give a permanentlydispersed product.

6. A process for the preparation of a polymeric lower alkyl ester ofacrylic acid in granular form which comprises dispersing andpolymerizing while thus dispersed a monomeric lower alkyl ester ofacrylic acid in a non-solvent therefor, by means of a colloid selectedfrom the group consisting of a soluble starch, gum tragacanth, gumacacia, gum tragon, water soluble glycol cellulose, sodium alginate',agar agar, glue and gelatin and stirring, the concentration of thecolloid being not substantially greater than an amount sumcient tomaintain the ester in the dispersed phase while stirring is continuedand to prevent coalescence of the dispersed particles duringpolymerization; the quantity of dispersing agent also being insufficientto give a permanently dispersed product. I

7. In a process for the preparation a polymeric lower alkyl ester ofacrylic acid in granular form the step which comprises maintaining adispersion of the monomeric lower alkyl ester of acrylic acid ester bymeans of a colloid selected from the group consisting of a solublestarch, gum tragacanth, gum acacia, gum tragon, water soluble glycolcellulose, sodium alginate, agar agar, glue and gelatin and stirringthruout polymerization, the concentration of colloid and the rate ofstirring being such that upon cessation of stirring beforepolymerization has been eifected substantially all of the monomericlower alkyl ester of acrylic acid will quickly coalesce to the liquidphase and upon cessation of stirring after polymerization the acrylicacid ester polymer will freely settle as a granular product.

8. In a process for the preparation of methyl acrylate polymer ingranular form the step which comprises maintaining the methyl acrylatemonomer in the dispersed phase by means of a colloid selected from thegroup consisting of a soluble starch, gum tragacanth, gum acacia, gum.tragon, water soluble glycol cellulose, sodium alginate, agar agar,glue and gelatin and stirring thruout the polymerization reaction, theconcentration of the colloid and the rate of stirring being such thatupon cessation of stirring before polymerization has been effectedsubstantially all of the methyl acrylate monomer will quickly coalesceto the liquid phase, and upon cessation of stirring 'afterpolymerization the methyl acrylate polymer will freely settle as agranular product;

9. In a process for the preparation of methyl acrylate polymers ingranular form the steps which comprise polymerizing in'a liquid mixturecontaining methyl acrylate and a non-solvent therefor in the presence ofa sufficient quantity of a colloid selected from the group consisting ofa soluble starch, gum tragacanth, gum acacia, gum tragon, water solubleglycol cellulose, sodium alginate, agar agar, glue and gelatin whilemaintaining a proportional amount of stirring to give a product theparticles of which will not coalesce to product a solid duringpolymerization, the amount of colloid and the amount of stirring beinginsumcient to give a permanently emulsified product.

10. In a process for the preparation of methyl acrylate polymers ingranular form the steps which comprise polymerizing in an aqueous mediumcontaining monomeric methyl acrylate, benzoyl peroxide and in thepresence of a sufficient quantity of colloid selected from the groupconsisting of a soluble starch, gum tragacanth, gum acacia, gum tragon,water soluble glycol cellulose, sodium alignate, agar agar, glue andgelatin while maintaining a proportional amount of stirring to give aproduct the particles of which do not coalesce to produce a solid duringpolymerization, the amount of colloid and the amount of stirring beinginsufllcient to give a permanently emulsified product.

11. In a process for the preparation of methyl acrylate polymers ingranular form the step which comprises polymerizing the methyl acrylatein a mixture containing approximately one part of methyl acrylate tofour parts of water, and approximately 0.2% to 3.0% of a colloidselected from the group consisting of a soluble starch,

" gum tragacanth, gum acacia, gum tragon, water soluble glycolcellulose, sodium alginate, agar agar, glue and gelatin, whilemaintaining a moderate amount of stirring which is sumcient to give aproduct the particles of which do not coalesce to produce a solid duringpolymerization, altho not so great that a permanently emulsified productis obtained upon complete polymerization.

12. A process for the preparation of a polymeric lower alkyl ester ofacrylic acid in granular form which comprises preparing a liquid mixturecontaining a monomeric lower alkyl ester of acrylic acid ester, 9.non-solvent therefor, and a colloid selected from the group consistingof a soluble starch, gum tragacanth. gum acacia, gum tragon, watersoluble glycol cellulose, sodium alginate, agar agar, glue and gelatin,stirring the resulting mixture to give a dispersion such that uponcessation of stirring the particles tho monomer will substantiallyimmediately coalesce, heating the dispersed mixture until polymerizationis complete, and subsequently stopping the stirring, and separating bydecantation the free settling granular acrylic acid ester polymer fromthe dispersing medium.

13. A process for the preparation of methyl acrylate polymers ingranular form which comprises preparing a liquid mixture containingmonomeric methyl acrylate, a non-solvent therefor, and a colloidselected from the group consisting of a soluble starch, gum tragacanth,gum acacia, gum tragon, water soluble glycol cellulose, sodium alginate,agar agar, glue and gelatin, stirring the, resulting mixture to give adispersion such that upon cessation of stirring the particles of monomerwill substantially immediately coalesce heating the dispersed mixtureuntil polymerization is complete, and subsequently stopping the stirringand separating by decantation the free settling granular methyl acrylatepolymer from the dispersing medium.

14. A process for the preparation of a lower alkyl ester of acrylic acidin granular form which comprises dispersing a monomeric acrylic acidester in water by means of a dispersing agent present in an amount notsubstantially greater than the amount necessary to maintain the ester inthe dispersed phase while stirring is continued and to preventcoalescence of the dispersing particles during polymerization, thequantity of dispersing agent also being insufiicient to give apermanently dispersed product, the polymerization being initiated byraising the temperature to approximately 75 C. and maintaining thetemperature during polymerization between approximately 75 C. and 82 C.until polymerization is complete.

15. In a process for the preparation of polymeric methyl acrylate ingranular form the step which comprises polymerizing the methyl acrylatein a mixture containing approximately one part of methyl acrylate tofour parts of water, and approximately 0.2% to 3.0% of a colloidselected from the group consisting of a soluble starch, gum tragacanth,gum acacia, gum tragon, water soluble glycol cellulose, sodium alginate,agar agar, glue and gelatin while maintaining a moderate amount ofstirring which is suflicient to give a product the particles of which donot coalesce to produce a solid during polymerization altho not so greatthat a permanently emulsified product is obtained upon completepolymerization, the polymerization being initiated by raising thetemperature to approximately 75 C. and during polymerization beingmaintained at a temperature between approximately 75 C. and 82 C.

JOHN WILLIAM CROOM CRAWFORD. JAMES MCGRA'I'H.

